Refractometer

ABSTRACT

The invention relates to a portable refractometer comprising a depression ( 7 ) for samples, located on an insertion tip ( 11 ) in such a way that once the insertion tip ( 11 ) has penetrated a liquid or a fruit, a sufficient quantity of the sample liquid remains in the depression ( 7 ) for samples, thus wetting a measuring surface ( 4 ) that is delimited in said depression by a transparent body. The refractive index of the wetting liquid can be determined by measuring the intensity of an optical beam that is reflected by the measuring surface ( 4 ).

[0001] The present invention relates to a refractometer for determiningthe refractive index of a liquid and, optionally, variables derivedtherefrom such as, for example, a sugar concentration, having a sensorsystem which has a radiation source for generating a measuring beam, abeam detector for detecting the measuring beam, and a measuring path tobe traveled by the measuring beam, on which path a measuring surface tobe wetted by the liquid and which interacts with the measuring beam issituated.

[0002] Such refractometers are used as digital or analog measuringinstruments, for example, for determining the concentration of certainsubstances dissolved in a liquid and influencing the refractive index.For example, in wine-making, the determination of the sugar content inthe grape juice is an important area of application of suchmeasurements.

[0003] Two types of measuring instruments are basically known in thisarea; one requires a sample to be taken and introduced into themeasuring instrument, for example, drop by drop, while the other designprovides for the immersion of a sensor into the analyte, i.e., into theliquid, to perform the measurement.

[0004] Sampling in the case of the first above-named variant isrelatively complicated and time-consuming, and the instrument must bethoroughly cleaned before each sampling. In the instrument of the secondvariant, the problem often arises that the refractometer has atemperature which is different from that of the analyte, so that therequired temperature compensation in determining thetemperature-dependent refractive index is very difficult to perform.Furthermore, it may be difficult to take into account the effect ofexternal light in optical measurements when a probe is introduced intothe substance to be analyzed.

[0005] U.S. Pat. No. 5,859,696 describes a refractometer of a typesimilar to the one mentioned previously for determining the sugarcontent in a liquid, in which an optical beam is emitted by a radiationsource, is reflected inside a transparent body on a measuring surface,and returned into a beam detector. If the measuring surface is wetted onthe outside with a liquid having a high refractive index, for example, asoft drink having a high sugar concentration, this results in therefractive index of the liquid on the measuring surface approaching therefractive index of the transparent body and thus in a weaker reflectionof the beam on the measuring surface or, in other words, in most of thebeam penetrating the measuring surface and entering the liquid, so thatthe intensity of the reflected beam is greatly reduced. This is detectedoptically and a high or low sugar concentration is obtained as afunction of the measured intensity of the reflected beam. The instrumentis usable in a simple manner by immersing a measuring tip into theliquid and detecting the corresponding signal on the portableinstrument. The temperature of the liquid is not taken into account inthe measurement.

[0006] In contrast, the object of the present invention is to refine arefractometer of a simple construction of the above-mentioned type insuch a way that simple and rapid handling is made possible, whileeffective temperature compensation is ensured.

[0007] This object is achieved according to the present invention by thefact that the measuring surface is situated in a depression for samplesof an insertion probe which is insertable into the liquid. The designaccording to the present invention makes it possible to use theinsertion probe, which is insertable into the liquid and containing thedepression for samples, for sampling and measurement, the refractionnumber, i.e., the refractive index, being determined in the depressionafter sampling the liquid to be measured. Only a small volume of liquidremains in the depression for samples; therefore the temperature betweenthe insertion probe and the liquid is rapidly equalized, so that therefractometer and the liquid have the same temperature at the time ofthe measurement. Furthermore, temperature compensation is made simple bymeasuring the temperature at the probe. For this purpose, a temperaturesensor may be placed within the insertion probe.

[0008] To take a new sample of the same or a different liquid, it issufficient to insert the insertion probe into a liquid again;optionally, the depression for samples may be briefly cleaned beforehandif this seems to be necessary when handling liquids to be measured.Otherwise, it is also conceivable to simply introduce the probe into theliquid again, the liquid measured in the first measurement being simplywashed out of the depression for samples by the second liquid.

[0009] Sampling and, optionally also, repeated use of the refractometeraccording to the present invention in a measurement is considerablysimplified, and effective temperature compensation is made possible. Forexample, the insertion probe may have a tip at its end to make insertioninto relatively large fruits possible to directly measure the fruitjuice inside. The surface of the insertion probe may also have a groovethrough which the liquid to be measured may flow into the depression forsamples after brief insertion through the liquid surface or into afruit.

[0010] An advantageous embodiment of the present invention provides forthe measuring surface to be delimited by a lens body.

[0011] The lens body is situated in such a way that one side is wettableby the liquid and on the other side the lens body surface is kept freeof the liquid. The radiation source and the beam detector are thenplaced on the side of the lens body which is kept free of the liquid,permitting the measuring beam from the radiation source to impinge onthe lens body, to be at least partially reflected there on the measuringsurface wetted with the liquid, and to be subsequently directed to thebeam detector. The intensity of the reflected measuring beam is then afunction of the ratio between the refractive index of the lens body andthat of the wetting liquid. The design of the geometry of the lens bodyis preferably such that the measuring beam is bundled or remains bundledin the lens body, and a suitable arrangement of the radiation source andthe beam detector with respect to the lens body may be selected.

[0012] According to another advantageous embodiment of the presentinvention, the measuring surface is delimited by a glass body.

[0013] In principle, the fact that the measuring surface is delimited bya glass body means that cleaning of the measuring surface afterperforming the measurement is simplified without scratching themeasuring surface. In addition, effective temperature equalizationbetween the sensor system and the liquid is ensured by the relativelygood thermal conductivity of glass.

[0014] With respect to the stability of the mechanical construction, thepresent invention is advantageously designed in such a way that theradiation source, the beam detector, and the lens body or, as the casemay be, the glass body, are held in a metallic mount, which is made ofsteel or aluminum in particular.

[0015] The design of the mount made of a stable material in the form ofsteel or aluminum results in the geometrical configuration of theradiation source, detector, and lens being sufficiently stable to thepoint that even impacts will not alter the measuring path. The metallicdesign also makes rapid and effective temperature equalization betweenthe individual elements of the sensor system and the liquid in thedepression for samples possible. Temperature equalization may beimproved by contact between the liquid and the metallic mount.

[0016] It is also advantageous to integrate a temperature sensor intothe area of the sensor system. In this way the temperature of the sensorsystem and of the liquid may be measured at the same time as thetemperature-dependent refractive index after these temperatures haveadjusted to one another, which occurs after a few seconds. The measuredrefractive index of the liquid may then be recalculated to a normalizedtemperature in an analyzer, taking into account for the compensation themeasured temperature. Because only a single temperature is to be takeninto account with the sensor system according to the present invention,the calibration of the sensor system is also greatly simplified.

[0017] To protect the sensor system in the event of brief temperaturechanges, the metallic mount may be surrounded by a material, inparticular by a synthetic material whose thermal conductivity is lessthan that of the mount material.

[0018] Normally the sensor system is protected by a plastic sheath madeof a polymer or an elastomer, which of course leaves the depression forsamples and the measuring surface free. The liquid sample taken and thesensor system are largely temperature-equalized independently of theambient temperature due to the thermal contact in the depression forsamples, and the refractive index is measured at this temperature. Thistemperature is measured simultaneously inside the refractometer in thearea of the sensor system, preferably by a temperature sensor, in orderto be able to take temperature influences into account and to refer themeasurement result to a normalized temperature.

[0019] According to another advantageous embodiment of the presentinvention, the lens body or the glass body is [made of a] material

[0020] [text missing]

[0021] in particular greater than 2. The refractive index of the lens ispreferably 1.85.

[0022] A high refractive index of the glass body is advantageous whenliquids also having high dielectric constants or refraction indices areto be measured.

[0023] The depression for samples advantageously has a volume of lessthan a milliliter for a particularly rapid temperature equalization.

[0024] The sensor system may be advantageously designed in that theradiation source is formed by an infrared LED and the beam detector isformed by a semiconductor which is sensitive in the infrared range.

[0025] Interference by external light is normally very small in theinfrared range, and the components used operate reliably and relativelyunaffected by errors.

[0026] The refractometer advantageously has a lens body which has anarea having greater curvature, which faces the radiation source and thebeam detector, and an area having lesser curvature which delimits themeasuring surface.

[0027] This design of the lens body ensures optimal guidance of themeasuring beam and optimum design of the measuring surface in terms ofits metrological characteristics, the measuring surface also being easyto clean.

[0028] The present invention is elucidated in the following on the basisof an exemplary embodiment illustrated in the drawing.

[0029]FIG. 1 schematically shows the internal structure of the sensorsystem;

[0030]FIG. 2 schematically shows an insertion probe, and

[0031]FIG. 3 shows an analyzer of the refractometer according to thepresent invention.

[0032] Initially the operating principle of the refractometer accordingto the present invention will be elucidated on the basis of FIG. 1. Thesensor system having radiation source 1 in the form of an infrared LED,beam detector 2 in the form of a light-sensitive semiconductor diode andthe measuring path between them is schematically illustrated. Ameasuring beam is emitted from radiation source 1 to glass lens body 3and enters, through its spherical or approximately spherical surface,perpendicularly into the glass body, through which it propagates tomeasuring surface 4, which is formed by a flat delimiting surface oflens body 3. The measuring beam is reflected or partly refracted thereinto the liquid as a function of the ratio between the refractivenumbers (refractive indices) of the material of the lens body and thematerial of liquid 5 which wets the lens body.

[0033] At least one portion of the beam may be reflected on measuringsurface 4 to beam detector 2 and detected by the latter.

[0034] The detected radiation intensity is measured using beam detector2; it is a measure of the refractive index of liquid 5. The measurementis compared to a reference measurement, which has been made eitherwithout a wetting liquid on lens body 3 or using a known liquid, fordetermining the refractive number.

[0035] Due to the small aperture angle of beam detector 2, very littlelight reaches beam detector 2 from the outside through wetting liquid 5and measuring surface 4, which permits the influence of external lighton the measurement to be kept particularly low.

[0036] Due to the orientation of the lens, the measuring beam sufferslittle loss in entering into lens body 3 and exiting to the beamdetector, while the flat design of measuring surface 4 makes the entryof external light into lens body 3 difficult. Cleaning of measuringsurface 4 on the outside, which is exposed to liquids 5 to be measuredis also facilitated by its flat design. The lens has typically adiameter of 3 mm and a refractive index greater than 1.5, in particulargreater than 2.

[0037] Radiation source 1 and beam detector 2 are each situated in boreholes within mount 6, which fixedly and reliably determine theirposition with respect to one another and to lens body 3.

[0038] In addition, mount 6, which is made of metal, for example, steelor aluminum, ensures excellent heat conduction, so that elements 1, 2, 3of the sensor system are reliably kept at the same temperature as mount6, and the small size of the sample of liquid 5 ensures that the sampleis very rapidly brought to the same temperature as mount 6 via heattransport thanks to glass lens body 3.

[0039]FIG. 1 shows, as an example, a small sample in the form of a dropof a liquid 5 on lens body 3; depression for samples 7 may also beregularly filled to the rim. In any case, temperature equalizationbetween the sample and mount 6 takes only a few seconds. The depressionfor samples has a volume of less than 1 mL, in particular less than 0.5mL.

[0040] Mount 6 is also provided with a temperature sensor 8 fortemperature measurement, which permits temperature compensation when themeasurements are analyzed.

[0041] Mount 6 is provided with a plastic layer 19, which insulates itthermally and thus protects the sensor system against varying externalconditions. In the edge area of lens body 3, the lens body is sealedwith respect to plastic layer 19 and mount 6 by elastic seals 20.

[0042] The schematic sectional view of FIG. 1 corresponds to a sectionalong broken line A-A in FIG. 2, which is described in the following.FIG. 2 shows an external view of a portable refractometer having ahandle 9, into which a digital display 10 is integrated. An analyzingdevice which analyzes the data delivered by beam detector 2 andtemperature sensor 8 is mounted in the body of the portablerefractometer. The sensor system is situated in the proximity ofinsertion tip 11, underneath depression for samples 7. In FIG. 2, lensbody 3 is shown in the form of a circle.

[0043] Insertion tip 11 is designed so that it may be stuck into a fruitso that the fruit juice contained in the fruit enters depression forsamples 7. However, it is also conceivable that insertion tip 11 isdipped into a liquid and that it has a groove-type notch on the topwhich leads to depression for samples 7 and allows the sample liquid toflow into depression for samples 7 even without insertion tip 11 beingdeeply dipped into the liquid or stuck into the fruit. Otherwise theinsertion tip is introduced into the substance to be analyzed as deep asrequired for the depression for samples to be filled.

[0044] The mode of operation of the refractometer is now brieflyelucidated schematically with reference to FIG. 3. Measuring beam 12 isgenerated by radiation source 1 in the form of an infrared beam, whichpropagates in the schematic representation of FIG. 1 along the centralaxis of mount hole 13, represented by a dot-and-dash line, in radiationsource 1 to measuring surface 4, reflected there, and continues fromthere along the central axis of mount hole 14, also represented by adot-and-dash line, in beam detector 2. The intensity of the reflectedmeasuring beam 12 is measured in beam detector 2. The measured intensityis supplied to analyzer 15, where a value of the refractive index, i.e.,refractive number, present is computed by a first computing device 16,initially without taking into account the temperature, using referencevalues. This value computed from the measurement is then referred to areference temperature and thus compensated for the influence oftemperature in second computing device 17, taking into account thetemperature value measured by temperature sensor 8 and also provided byanalyzer 15. The value of the refractive index, i.e., refractive number,computed and corrected in this way is then supplied to a display 18 andthere output to the user via a digital display.

What is claimed is:
 1. A refractometer for determining the refractiveindex of a liquid (5) and, optionally, variables derived therefrom suchas, for example, a sugar concentration, having a sensor system which hasa radiation source (1) for generating a measuring beam (12), a beamdetector (2) for detecting the measuring beam (12), and a measuring pathto be traveled by the measuring beam, on which path a measuring surface(4) to be wetted by the liquid (5) and which interacts with themeasuring beam (12) is situated, wherein the measuring surface (4) issituated in a depression for samples (7) of an insertion probe (11)which is insertable into the liquid (5).
 2. The refractometer as recitedin claim 1, wherein the measuring surface (4) is delimited by a lensbody (3).
 3. The refractometer as recited in claim 1 or 2, wherein themeasuring surface (4) is delimited by a glass body.
 4. The refractometeras recited in claim 2 or 3, wherein the radiation source (1), the beamdetector (2), and the lens body (3) or, as the case may be, the glassbody, are held in a metallic mount (6), which is made of steel oraluminum in particular.
 5. The refractometer as recited in one of thepreceding claims, wherein the metallic mount (6) is surrounded by amaterial, in particular by a synthetic material (19) whose thermalconductivity is less than that of the mount (6) material.
 6. Therefractometer as recited in claim 1 or one of the subsequent claims,wherein a temperature sensor (8) is provided in the area of the sensorsystem (1, 2, 3).
 7. The refractometer as recited in claim 2 or one ofthe subsequent claims, wherein the lens body (3) or the glass body ismade of a material having a refractive index which is greater than 1.5,in particular greater than
 2. 8. The refractometer as recited in claim 1or one of the subsequent claims, wherein the volume of the depressionfor samples (7) is less than 1 milliliter.
 9. The refractometer asrecited in claim 1 or one of the subsequent claims, wherein theradiation source (1) is formed by an infrared LED and the beam detector(2) is formed by a semiconductor which is sensitive in the infraredrange.
 10. The refractometer as recited in claim 1 or one of thesubsequent claims, having a lens body (3) which has an area havinggreater curvature and an area having lesser curvature of its surface,wherein the area having greater curvature faces the radiation source (1)and the beam detector (2), and the area having lesser curvature delimitsthe measuring surface (4).
 11. A method for operating a refractometer asrecited in claim 1 or one of the subsequent claims, wherein theinsertion tip is initially dipped into the sample liquid or insertedinto a fruit and then the insertion tip is removed, and the refractiveindex is measured on the sample of the sample liquid remaining in thedepression for samples.